1. Field of the Invention
This invention relates to novel metal oxides having perovskite-type crystal structures and containing metals of the platinum group. These oxides are catalytically active, especially in heterogeneous gas-phase reactions like those involved in the oxidation-reduction purification of the exhaust gases from internal combustion engines.
2. The Prior Art
Much effort has been expended in recent years in developing improved heterogeneous catalysts for the oxidation of volatile carbon compounds in air and for the reduction of nitrogen oxides to nitrogen by hydrogen, carbon monoxide, and other carbon compounds. Such efforts have been directed toward the reduction of atmospheric pollution by industrial processes but have also been largely directed toward the reduction of atmospheric pollution by exhaust gases from internal combustion engines. Catalysts useful in such processes will desirably be low in cost, selective in promoting desired oxidation and/or reduction reactions at relatively low temperatures, active for long periods at the temperatures involved and in the presence of the materials incidental to these reactions, simple to prepare in suitable forms having high catalytic activity, active at relatively low surface areas per unit weight of catalytic material, and will also desirably have other properties well recognized in the art.
Among the catalytic materials proposed for use in promoting chemical reactions such as those involved in the purification of automotive exhaust gases are catalysts containing the platinum metals ruthenium, osmium, rhodium, iridium, palladium, and platinum. Such catalytic materials are relatively expensive; require sometimes impractically large amounts of scarce materials; frequently must be prepared by carefully controlled processes for optimum catalytic activity; are relatively short-lived, apparently because of either the formation of relatively volatile oxides (osmium and ruthenium), because of changes in crystallite particle size or surface properties, or because of interaction with various components of exhaust gases in ways which reduce their catalytic activity (for instance by forming catalytically less active compounds or alloys and by forming volatile halide compounds); and are unsatisfactory in other ways.